Apparatus for fractionating fiber suspensions in accordance with fiber length

ABSTRACT

A slurry of mixed paper making fibers of a wide range of lengths is fractionated into two fractions wherein the fibers are predominantly of lengths greater or less, respectively, than a predetermined intermediate control length, by feeding the slurry under pressure into the inlet end of a cylindrical screen member having slotted perforations of predetermined size and spacing in accordance with the control fiber length for the slurry to be fractionated, i.e. the slots are substantially less wide than the control fiber length and they are spaced to provide lands therebetween which are also less wide than the control fiber length. The supply flow is caused to circulate circumferentially of the wall of the screen member so that small fibers and other particles will pass through the slots to form the short fiber fraction, but fibers longer than the control fiber length will either flow past the slots or will fold over or &#39;&#39;&#39;&#39;hair-pin&#39;&#39;&#39;&#39; on the lands between slots. Periodic pulsations through the slots dislodge hair-pinned fibers for return to the supply flow and ultimate discharge from the exit end of the screen member to form the long fiber fraction.

United States Patent Seifert APPARATUS FOR FRACTIONATING FIBER SUSPENSIONS IN ACCORDANCE WITH FIBER LENGTH Peter Seifert, Middletown, Ohio [75] Inventor:

[73] Assignee: The Black Clawson Company,

Middletown, Ohio [22] Filed: Aug. 28, 1973 [21] Appl. No.: 392,263

[52] US. Cl. 209/240; 209/273; 209/306; 209/397 [51] Int. Cl. B07B 1/20 [58] Field of Search 209/273, 303-306,

[56] References Cited UNITED STATES PATENTS 1,037,597 9/1912 Coppace 209/270 2,975,899 3/1961 Cannon 209/273 3,276,584 10/1966 Mathewson... 209/270 X 3.34991 1 10/1967 Reddick 209/304 3,404,065 10/1968 lngmarsson... 209/270 X 3,420,373 l/l969 Hunter 209/306 X 3,446,354 5/1969 SkardaL. 209/306 X 3,680,696 8/1972 Morin 209/306 X 3,713,541 l/1973 Nelson... 209/273 3,726,401 4/1973 Bolton 209/273 X Germany 209/304 563,239 9/1923 France 209/304 Primary E.\'aminerRobert Halper Attorney, Agent, or Firm-French & Bugg Biebel [57 ABSTRACT A slurry of mixed paper making fibers of a wide range of lengths is fractionated into two fractions wherein the fibers are predominantly of lengths greater or less, respectively, than a predetermined intermediate control length, by feeding 'the slurry under pressure into the inlet end of a cylindrical screen member having slotted perforations of predetermined size and spacing in accordance with the control fiber length for the slurry to be fractionated, ie. the slots are substantially less wide than the control fiber length and they are spaced to provide lands therebetween which are also less Wide than the control fiber length. The supply flow is caused to circulate circumferentially of the wall of the screen member so that small fibers and other particles will pass through the slots to form the short fiber fraction, but fibers longer than the control fiber lengthwill either flow past the slots or will fold over or hair-pin" on the lands between slots. Periodic pulsations through the slots dislodge hair-pinned fibers for return to the supply flow and ultimate discharge from the exit end of the screen member to form the long fiber fraction.

3 Claims, 3 Drawing Figures U.S. Patent Sept. 30,1975 3,909,400

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APPARATUS FOR FRACTIONATING FIBER SUSPENSIONS IN ACCORDANCE WITH FIBER LENGTH BACKGROUND OF THE INVENTION This invention relates to the classification of mixtures of fibers of varied lengths, such particularly as papermaking fibers, into fractions in accordance with the lengths of the fibers composing each fraction, e. g. fractions wherein the fibers are predominantly of lengths greater or less than a predetermined intermediate length, respectively. For example, the invention is applicable to the fractionation of mixed pulp into a long fibered, high freeness pulp, and a short fibered, low freeness pulp.

The prior art has approached this objective as a screening operation, i.e. a process of separating long and short fibers in accordance with whether or not they pass through holes of particular size in a screen member. For example, the paper Groundwood Pulp Fractionation and Screening with Pressure Screens at High Consistency by A. J. Sternby and D. F. Lehman (TAPPI 44, No. 6:401, June 1961) described a series of tests of such a fractionating process in a screen constructed in accordance with Martindale U.S. Pat. No.

2,835,173 wherein the cylindrical screen member had multiple round holes 0.050 inch or 0.062 inch in diameter, and the rotating vanes operated at a close clearance from the screen member, e.g. 0.030 inch or less.

The Sternby-Lehman paper is the most comprehensive prior art discussion of fiber fractionation known to the present inventor, although a list of prior art patents of some relevance to this subject should include Szepan et al. U.S. Pat. No. 3,074,553, Dick U.S. Pat. No. 3,223,239, Nelson U.S. Pat. No. 3,533,505, Salomon U.S. Pat. No. 3,561,663, Sutherland U.S. Pat. No. 3,547,267 and Morin U.S. Pat. No. 3,680,696, as well as Trimby U.S. Pat. Nos. 1,856,176 and 1,978,433. All of these patents have the characteristic already noted, namely, to the extent that they consider fiber fractionation, they do so as a screening operation wherein hole size and shape control the split between long and short fibers, as well as between the preferred, clean fraction and the less desired, less clean fraction.

SUMMARY OF THE INVENTION The present invention has as its primary objective the establishment of a new and different approach to the fractionation of mixed fibers into groups of predominantly long and short fibers respectively, and more particularly to establish principles of operation for fractionation which will enable accurate control of the intermediate fiber length at which the split occurs between the two fractions.

A more specific object of the invention is to establish new principles for effecting fiber fractionation which will make it possible to predetermine the fiber length at which a fiber mixture is split into two fractions, and to carry out fractionating operations with a maintained high degree of consistency of the distribution of fiber lengths in the resulting two fractions with respect to the desired division point.

In accordance with the invention, fiber fractionation is performed by apparatus similar in structure and mode of operation to a pressure screen and incorporating a cylindrical screen member provided with multiple slots extending substantially axially thereof, but

wherein the size of the slots is of less importance than their relative spacing and their radial dimension, and wherein the separating action is due primarily to operating conditions which cause long fibers to fold over of hair-pin on the lands between adjacent slots rather than to the inability of the long fibers to flow through the slots.

In a typical application of the invention, the stock to be fractionated will comprise fibers of a wide range of lengths such as will occur, for example, in groundwood pulp or waste corrugated board pulp, and the primary objective of treatment in accordance with the invention is to produce a fraction consisting predominantly of the long fibered constituent of the mixture. Such pulp is introduced into one end of a cylindrical chamber having one wall, preferably the outer wall, defined by a cylindrical screen member having multiple axially extending slots therethrough, and the stock is caused to circulate in an approximately helical pattern so that it will travel from the inlet end of the chamber to its exit end. The

apparatus of which the screen member forms a part will also include a primary outlet for stock which passes radially through the slots in the screen member, and a secondary outlet for continuously removing stock which has traversed the length of the chamber and been rejected by the screen member.

In accordance with the invention, the slots in the screen member are substantially longer than they are wide, and their width and spacing are selected in accordance with the intermediate fiber length (control fiber length) at which it is desired to have the fractionation take place. More specifically, the width of each slot should be substantially less than the control fiber length, but these slots should be spaced relatively close together so that the width of each of the lands between adjacent slots is less than the control fiber length. There is also a relationship between land width and the thickness of the screen member, which is the same as the radial dimension of each slot, this relationship being that the sum of the width of one land plus the thickness of the screen member should preferably be less than the control fiber length.

In the practice of the invention with apparatus constructed as summarized above, the stock to be fractionated is caused to circulate circumferentially of the screen member under such conditions of speed and hydraulic shear that the fibers, and particularly the longer fibers, tend to be oriented generally tangentially of the slotted cylindrical wall. Oriented long fibers which are longer than the width of a slot therefore have a greater tendency to flow past the slots than through a slot, and even if the leading end of a long fiber enters a slot, its trailing portion will have a greater tendency to fold or hair-pin over the land on the off running side of the slot than to pass through the slot, due to the hydraulic forces urging the fiber circumferentially of the screen member. On the other hand, fibers of shorter lengths, approximating or less than the width of a land, will have a greater tendency to pass through the slots, and the majority of the non-fibrous particles which constitute dirt and are small enough to pass through a slot will do In addition to the circulatory force applied to the stock within the screen chamber, sufficient force is applied axially of the screen member to maintain a constant flow towards its exit end where reject is accumulated in conventional operation of a screen of this type.

Further, pulsations are applied through the slots periodically which have the effect of dislodging hair-pinned fibers and causing them to return to the circulatory flow. A relatively high volumetric rate of discharge flow is maintained from the exit chamber to minimize recirculation of long fibers into the screen chamber, and this flow will therefore contain the desired high proportion of long fibers and relatively low proportions of short fibers and other small particles.

The principles of the invention make it possible to establish with reasonable accuracy the value of the control fiber length at which the split occurs between the two fractions. It is of particular interest and value that this result can be accomplished by suitable selection of physical factors, namely slot width, slot spacing and screen cylinder thickness, relatively independently of such factors as consistency, flow rate and pulp freeness which are very important to the prior art approaches to fiber fractionation. More specifically, and as previously noted, the width of each slot should be substantially less than the control fiber length, and preferably less than one-half the control fiber length, and the spacing of the slots should be such that the width of each land is of the same order as or somewhat less than the control fiber length.

The importance of the relation between land width and the radial dimension of the slots derives from the fact that if the walls of the slots are parallel, it is possible for viscous drag to draw a hair-pinned fiber through the slot, and this effect would increase with slot depth. In principle, therefore, the sum of land width plus slot depth should preferably be less than the control fiber length, but it is possible to minimize the importance of this factor by relieving the slots on the exit side of the screen member so that their walls diverge in the direction of flow from the screen chamber, thereby reducing their effective length to zero or a minor fraction of the wall thickness of the screen member.

In summary, therefore, the primary importance of slot width is that the slots should be sufficiently narrow to prevent passage therethrough of a fiber of desired length which is aligned across the slot, and slot width accordingly has a governing effect on the length of the retained fibers and on the size of the non-fibrous particles which are accepted by the slots.

In the practice of the invention, the most important factor is the combination of slot spacing and screen cylinder thickness, because that governs the condition of hair-pinning of fibers. If, for example, both slot spacing and cylinder thickness are small, a substantial proportion of relatively short fibers can be expected to hairpin and thus be retained on the inlet side of the cylinder. For increased spacing and/or thickness, such fibers are more likely to pass through the slots while longer fibers will still have a greater tendency to hair-pin and to be retained on the long fiber side. The invention therefore provides principles by which mechanical factors can be preselected to establish the division point between two fractions of a fiber mixture, relatively independently of other operating conditions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view, partly broken away, illustrating fiber classifying apparatus constructed in accordance with the invention; and

FIGS. 2 and 3 are enlarged fragmentary views illustrating diagrammatically the operation of the apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The apparatus shown in the drawing is constructed generally in accordance with the Martindale patent, with certain exceptions in accordance with the invention. It comprises a main housing on a base 11, and in the upper end of the housing is an inlet chamber 12 having a tangential inlet port 13 to which the furnish supplied under pressure is as customary with such screening apparatus. A cylindrical screen member 15 divides the interior of the housing below chamber 12 into a central screen chamber 16 and an accepts chamber 17 having an outlet port 18.

The bottom wall 20 of the accepts chamber 16 separates the chamber 16 from a collection chamber 21 leading to a discharge port 22 provided with a control valve assembly 23 which can be preset to provide a desired continual flow of stock which has passed through the screen chamber 16. Heavy particles which settle into the chamber 21 drop therefrom to the heavy trash collection box 24 by way of manually controlled valve 25.

A rotor is supported on a drive shaft 31 in the center of the supply chamber 16 and is driven through suitable gearing or belts by a motor 33 also mounted on the base 11. Vanes or bars 35, shown as of the same configuration as in the Martindale patent, are mounted on the rotor 30 by support rods 36. Adjustable connections 37 between the inner ends of rods 36 and rotor 30 provide for positioning the vanes 35 in properly spaced relation with the inner surface of screen member 15, a space in the range of 3/16 to V2 inch being preferred depending upon circumferential speed for the reasons already explained. The vanes 36 extend the full length of the screening surface of screen member 15, and they are helically curved and so arranged that the upper end of each vane is spaced forwardly of the lower end in the direction of rotation of the rotor, shown as clockwise. Two vanes 35 are shown, but other numbers can be used, and in general a greater number, e.g. four, provide optimum operation.

The screen member 15 is provided with multiple screening slots 40 shown as extending generally parallel with its axis and the axis of rotor 30, i.e. so that they extend approximately at right angles to the somewhat helical path of the stock circulating therepast. These slots are shown as arranged in circumferential rows, with the individual slots being 2 inches in length and the rows spaced 1 inch apart axially of member 15, but neither of these dimensions nor the specific arrangement is critical, the important consideration being slot width, spacing and depth as described above.

In general, fibers useful for papermaking purposes have lengths ranging from 0.5 to 6 mm, but the longer the fiber, the greater strength-it will impart to the paper. Fiber fractionation is practiced for the purpose of obtaining one fraction which will be more valuable than the initial mixture because it contains predominantly long fibers and will therefore be of maximum strength properties and high qualities, while the other fraction will comprise predominantly short fibers and will therefore be of lower strength properties and lower quality. Since the value of the first fraction will also be enhanced by cleanliness, it is desirable to combine fractionation with cleaning, by concentrating as much contaminant material as possible in the short fiber fraction. These considerations in combination dictate the dimensions of the slots within the limits imposed by the desired control fiber length.

The principles of operation of the invention as discussed above are illustrated in diagrammatic fashion in FIG. 2, which represents an enlarged fragment of screen member comprising slots 40 alternating with lands 42. In operation, as the stock travels clockwise in FIG. 2, long fibers 44 tend to be oriented generally tangentially of the slots 40, and therefore to travel past the slots rather than through them, notwithstanding the pressure differential which is maintained between the inlet and exit sides of the screen member. Fines and short fibers 45, as well as dirt particles 46 small enough to travel through the slots, will do so, by reason of this maintained pressure drop.

FIG. 2 also illustrates that when a long fiber 440 impinges on the screen member in such manner that its leading end enters the slot, its trailing portion will still tend to sweep past the slot so that the fiber hair-pins on the land 42 beyond the slot, as indicated at 44b. This action will occur repeatedly as the operation proceeds, but clogging of the screen is prevented, and retention of the long fibers is assured, by the pulsations through the slots which are developed periodically by the traveling vanes 35, in the manner described in the Martindale patent. In other words, the pressure wave which travels around the inlet side of the screen member with the leading edge of each vane will tend to cause small fibers and particles to travel through the slots but will merely cause hair-pinned fibers to remain in position. However, the negative pressure wave which travels with the trailing edge of each vane will tend to draw out of the slots whatever portion of a hair-pinned fiber has entered a slot and thus free the hair-pinned fibers for continued travel along the surface of the screen member. This action can occur a number of times for a given fiber, but ultimately it is more likely to reach the chamber 22 at the bottom of the screen member than to pass through a slot.

FIG. 2 also illustrates the importance of establishing the appropriate dimensional relations between slots 40, lands 42 and the fibers to be retained. More specifically, the slots 40 should be wide enough to accept at least a large majority of whatever dirt particles are present in the supply flow but should be substantially less in width than the control fiber length at which it is desired to split the stock into two fractions. As a typical example, if 2.2 mm is to be the control fiber length, sat- ?isfactory results have been obtained with slots 40 which 'are 0.5 mm in width and are spaced ten to the inch so -that each land 42 is approximately 2 mm in width. With these dimensions, fibers of the control length of 2.2 mm will tend to be allocated more or less evenly between the two fractions, the majority of the shorter fibers and other particles capable of passing through slots 40 will do so, but the great proportion of longer fibers will tend to remain on the inlet side of the screen member as part of the long fibered fraction discharged through outlet port 22.

FIG. 3 illustrates the principle of the relationship of land width to slot depth as discussed above. Each slot comprises a short straight-sided portion 50 on the inlet side of the screen member 15' and a substantially longer relieved portion on the exit side of the screen member defined by walls 51 which diverge away from the portion 50. The important slot depth dimension for the purpose of the invention is the radial extent of slot portion 50, which may be as little as 0.5 mm when the thickness of the screen member 15 as a whole is 3/16 inch. Under these circumstances, the slot depth may practically be disregarded, because of the relatively low force of viscous drag therethrough, and the widths of slot portion 50 and lands 52 will establish the control fiber length as described in connection with FIG. 2.

What is claimed is:

1. Apparatus for classifying a slurry of paper making fibers of a range of lengths into two fractions wherein the classified fibers are predominantly of lengths greater or less than a predetermined intermediate control length respectively, comprising:

a. a housing,

b. a cylindrical screen member within said housing chamber separating the interior thereof into a screen chamber and a discharge chamber on opposite sides of said screen member,

c. inlet means for supplying the slurry to one end of said screen chamber,

d. said screen member having multiple slots therethrough extending generally axially thereof and of substantially greater length than width,

i. the width of each said slot being substantially less than said control fiber length,

ii. said slots being spaced circumferentially of said screen member to provide a land between each pair of adjacent slots which is of a width less than said control fiber length,

primary outlet means for continuously removing from said discharge chamber slurry containing fibers and other particles which have passed through said slots,

f. means for causing the slurry to circulate generally circumferentially of said screen chamber and thereby to cause fibers of greater length than said control length to hair-pin on said lands rather than to pass through said slots,

. means for causing periodic pulsations through said slots to dislodge hair-pinned fibers from said lands for continued travel toward said other end of said screen chamber, and

h. secondary outlet means for continuously withdrawing slurry including such dislodged fibers from said other end of said screen chamber.

2. Classifying apparatus as defined in claim 1 wherein the radial dimension of said slots is such that the sum of said dimension and the width of one of said lands is less than said control fiber length.

3. Apparatus as defined in claim 1 wherein said screen chamber is located on the radially inner side of said screen member and said discharge chamber is on the radially outer side of said screen member. 

1. APPARATUS FOR CLASSIFYING A SLURRY OF PAPER MAKING FIBERS OF A RANGE OF LENGTHS INTO TWO FRACTIONS WHEREIN THE CLASSIFIED FIBERS ARE PREDOMINANTLY OF LENGTHS GREATER OR LESS THAN A PREDETERMINED INTERMEDIATE CONTROL LENGTH RESPECTIVELY, COMPRISING: A. A HOUSING, B. A CYLINDERICAL SCREEN MEMBER WITHIN SAID HOUSING CHAMBER SPARATING THE INTERIOR THEREOF INTO A SCREEN CHAMBER AND A DISCHARGE CHAMBER ON OPPOSITE SIDES OF SAID SCREEN MEMBER, C. INLET MEANS FOR SUPPLYING THE SLURRY TO THE ONE END OF SAID SCREEN CHAMBER, D. SAID SCREEN MEMBER HAVING MULTIPLE SLOTS THERETHROUGH EXTENDING GENERALLY AXIALLY THEREOF AND OF SUBSTANTIALLY GREATER LENGTH THAN WIDTH, I. THE WIDTH OF EACH SAID SLOTS BEING SUBSTANTIALLY LESS THAN SAID CONTROL THE FIBER LENGTH, II. SAID SLOTS BEING SPACED CIRCUMFERENTIALLY OF SAID SCREEN MEMBER TO PROVIDE A LAND BETWEEN EACH PAIR OF ADJACENT SLOTS WHICH IS OF A WIDTH LESS THAN SAID CONTROL FIBER LENGTH, E. PRIMARY OUTLET MEANS FOR CONTINUOUSLY REMOVING FROM SAID DISCHARGE CHAMBER SLURRY CONTAINING FIBERS AND OTHERS PARTICLES WHICH HAVE PASSED THROUGH SAID SLOTS,
 2. Classifying apparatus as defined in claim 1 wherein the radial dimension of said slots is such that the sum of said dimension and the width of one of said lands is less than said control fiber length.
 3. Apparatus as defined in claim 1 wherein said screen chamber is located on the radially inner side of said screen member and said discharge chamber is on the radially outer side of said screen member. 